1. Field of the Invention
The present invention relates to a method for the production of single-component polyurethane lacquers for metal-strap lacquering.
2. Description of the Prior Art
Polyurethanes would satisfy the essential requirements for use as coating for production of lacquered metal-strap goods. Thus they exhibit excellent surface hardness and resistance to scraping and chemicals. Further, polyurethanes, produced by means of aliphatic or cycloaliphatic polyisocyanates, yield lacquer films which are distinguished, in addition, by outstanding resistance to weathering.
Polyurethanes, however, are practically unused in the strap-lacquering sector since, on account of the reaction mechanism, they entail a two-component application which leads in metal-strap lacquering to great, in many cases unsurmountable difficulties. Moreover, it has thus far not been possible to achieve with sufficiently good surface hardness the good deformation values readily obtainable today with PVC-organosols and PVDF-systems. There has been no shortage of attempts to convert, especially for the general lacquering sector, the above two-component system into a pseudo-single component system. For this, in principle, the two following procedures have been described:
1. Resins with isocyanate end groups are produced on the basis of polyethers and polyesters. These resins after application react with the moisture in the air to form cross-linked, high molecular weight polyether-or polyester-polyureas. The second component in this case is thus water. From this fact arises also the drawbacks of this, in principle, elegant method. Care must be taken to exclude all water before application. Only water-free resins, solvent, pigments etc. are to be used. Since these requirements can be satisfied only with great technological difficulty, the use of such products has remained limited heretofore.
2. The second way proceeds from the general fact that many organic bonds tend to split up again from a certain temperature upwards. A series of substances has been found which form stable compounds with isocyanate groups at room temperature which split again into isocyanate groups at elevated temperatures between 100.degree. and 200.degree. C. as a rule. Such products are known as isocyanate yielders or as blocked or masked isocyanates in the technical literature. In the presence of hydroxyl groups no reaction of the isocyanate groups can occur at room temperature, so that the single-component process becomes possible.
It is also by method 2 of the state-of-the-art that 2-butanone oxime-blocked isocyanates are used as cross-links for hydroxylacrylate (of U.S. Pat. No. 3,694,389). The disadvantage of hydroxylacrylates as polyol components for polyurethane strap-coating lacquers is in the poor deformability. Good deformability is an indispensable prerequisite for the suitability of coatings for the metal-strap lacquering sector. Moreover, generally suitable polyols are polyethers and higher molecular weight epoxides. These two groups of products, however, on account of their poor weathering characteristics are not recommended for combination with aliphatic and cycloaliphatic isocyanates.
From DT-OS No. 23 46 818 are also known combinations of OH-group-containing polyesters with .epsilon.-caprolactam-blocked polyisocyanates. On account of the high necessary object temperature of about 240.degree. C., such lacquers are to be pigmented only with very thermostable pigments. Pigments of high thermostability, however, are not usable without reservation, because of toxicological reasons on account of their heavy metal base. If, however, the requisite object temperature can be reduced below 220.degree. C., plenty of pigments are available which are not hazardous to health.